Circuit controlling the total time to reach maximum current value in an inductive load



Sept. 23, 1969 J WQQDLEY ETAL 3,469,150

CIRCUIT CONTROLLING THE TOTAL TIME TO REACH MAXIMUM CURRENT VALUE IN AN INDUCTIVE LOAD Filed May 4. 1967 (L 1; Y LOAD POWER IN gu py he an \SER ag I6 18 r POWER SUPPLY n I +1ov INVENTORS FRANK .3 an GOOLEY lsuqm eorrRfiY EYLI km MF ALAG ATTORNEYS United States Patent 3,469,150 CIRCUIT CONTROLLING THE TOTAL TIME TO REACH MAXIMUM CURRENT VALUE IN AN INDUCTIVE LOAD Frank John Woodley, Knebwortll, and William Geolfrey Eyles, Stevenage, England, assignors to International Computers and Tabulators Limited, London, England, a British company Filed May 4, 1967, Ser. No. 636,141 Claims priority, application Great Britain, May 11, 1966, 20,851/ 66 Int. Cl. H01h 47/18 US. Cl. 317-123 Claims ABSTRACT OF THE DISCLOSURE A circiut for controlling accurately the time at which an inductive load, such as a print hammer electromagnet, is operated by a drive signal. The load is connected to a switch transistor and to an adjustable reference voltage. The reference voltage source maintains a constant voltage across the load during the initial build up of current in the load. Hence, variation of the reference voltage varies the total time taken for the current in the load to reach its maximum value.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to circuits for enerigising inductive load.

Description of the prior art It is well known to control the energisation of an inductive load, such as the winding of an 'electromagnet, by means of a transistor which acts as a current switch. It is necessary in some applications to control the movement of the armature with great accuracy. One such application is the operation of the electromagnets which drive the print hammers in line printers of the kind usually referred to as on-the-fly. The print wheels, or print barrel, of the printer is rotated at high speed and a particular character is printed by firing the print hammer, which is an extension of the armature of the electromagnet, at the precise moment when that character is passing the printing line. Any error in the timing of the hammer will result in displacement of the printed character above or below the printing line. Any error in impact velocity of the hammer results in differences of print density between characters. Hence, both timing and impact velocity must be controlled accurately.

United States patent specification 3,183,830 describes a print hammer mechanism and operating circuit in which the travel of the hammer is adjustable mechanically and the timing of the operation of the hammer relative to an initiating signal is adjustable electrically by a variable time delay circuit. The specification explains that adjustment of the travel of the hammer affects both the timing of the hammer and the impact velocity of the hammer. Thus, adjustment of this factor alone enables the print registration or the print density, but not both to be optimised. The additional electrical adjustment enables the registration to be optimised. However, one time delay circuit is necessary for each hammer, of which there are usually one hundred, to one hundred and fifty in typical line printers, so that the cost of the rather complex time delay circuit forms a substantial part of the total cost of the printer.

SUMMARY OF THE INVENTION According to the invention an electrical circuit includes first and second power supply lines; an inductive load; a

3,469,150 Patented Sept. 23, 1969 current limiting resistor and an electronic switch connected in series between said power supply lines; a source of first value of voltage connected between said power supply lines; means to apply an initiating signal to said electronic switch to change said switch from a non conductive to a conductive state; first means operative to maintain the voltage across the load at a substantially constant second value less than said first value of voltage for a period immediately following the change of the electronic switch from the non-conductive state to the conductive state; and said first means including second means operable to adjust the second value of voltage to control the rate of current rise in the inductive load.

BRIEF DESCRIPTION OF THE DRAWING FIGURE 1 is a block diagram illustrating the invention.

FIGURE 2 is a simplified diagram of a circuit embodving the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT An inductive load 1 (FIGURE 1) and a current limiting resistor 17 are connected in series with a switch 2 between a pair of power supply lines 3 and 4. The power supply lines are energised by a power supply 18. The switch 2 is normally open and can be closed for a particular interval by an initiating pulse applied over a line 5. The switch 2 may be, for example, a transistor which is normally maintained in a non-conductive state and is driven into a conductive state by the pulse on line 5. The switch may be, alternatively, a thermionic, or gas, valve or similar electronic switching device.

In the simple series circuit of switch and load, the voltage across the load 1 rises substantially instantaneously to the voltage between the supply lines 3 and 4, when the switch 2 is closed. The current increases approximately exponentially towards a value determined by the total resistance of the load, the resistor 17 and the switch. The voltage across the load will decrease towards a limiting value, as the current increases, determined by the resistance 1a of the load.

The invention employs a stabilising circuit 6 to hold the voltage across the load at a substantially constant voltage for an initial period following the closure of the switch 2. This voltage is less than the supply voltage and is adjustable to control the amount of delay in the energisation of the load. For example, the voltage across the supply lines 3 and 4 might be 30 volts and the voltage across the load might be 10 volts when the current has reached a steady value. If the stabilising circuit is set at 20 volts, for example, the voltage across the load will rise only to 20 volts when the switch 2 is closed. Consequently, the current will increase more slowly than if the full 30 volts were applied initially. The stablising circuit is arranged to set an upper limit only to the voltage across the load. The voltage across the load is held constant at 20 volts and the current rises linearly until it reaches a value such that the voltage drop across the resistor 17 is equal to the difference between the supply voltage and the stabiliser voltage. The stabilising then ceases to have any effect and for the remainder of the duration of the initiating pulse the current flowing is determined by the full 30 volt supply voltage. This ensures that the total energy which is supplied to the load does not change very much when the delay time is altered by adjustment of the stablising circuit.

The characteristics of the stabilising circuit may be achieved in a variety of ways. For example, a Zener diode may be connected so as to be effectively in parallel with the load. Adjustment of the delay time in steps may be provided by a switch which selects in turn Zener diodes having different operating voltages. This arrangement operates satisfactorily, but the diode may have to pass a substantial current if the load current is large. An alternative circuit in which the current rating of the diode is greatly reduced is shown in FIGURE 2.

Transistor T1 is normally fully conducting and T2 is non-conducting. An initiating pulse of fixed duration is applied to the base of transistor T1 over line 7 and makes T1 non-conducting. The voltage at the base of T2 is determined by the potential divider chain consisting of resistors 8 and 9 and Zener diode 10, which is connected between 35 volts supply line 3 and +10 volt supply line 11. T1 switching oif produces a voltage stop at the junction of the resistor 8 and the diode 10. This drives the transistor T2 into conduction and current starts to flow through resistor 12, inductive load 13 and resistor 14.

The voltage at the junction of resistor 8 and Zener diode 10 is substantially constant being the Zener voltage plus the forward emitter/base drop of transistor T2.

The current in resistor 8 is therefore constant and is chosen to be greatly in excess of the base drive required by transistor T2. The negative feedback loop of diode 15 and resistor 16 ensures that initially the current of resistor 8 is supplied almost entirely through resistor 16. Hence the junction of resistor 12, load 13 and diode 15 is at a constant +ve voltage with respect to the junction of resistor 8 and Zener diode 10, and hence is at a constant potential relative to line 3. The current in the inductive load 13 rises linearly until a level is reached when the volt drop across resistor 12 due to load current causes T2 to bottom. The voltage gain around the loop is now substantially zero. The voltage across the load 13 reverts to an exponential decay and diode 16 becomes reverse biased breaking the negative feedback loop.

When resistor 16 is zero the initial voltage across the load 13 is substantially equal to the volt drop across resistor 8, and the delay is a maximum. With resistor 16 at maximum value the initial voltage across load 13 is virtually the full power supply volts and the delay is a minimum.

If the inductive load 13 possesses some internal resistance 14 then the current waveform will depart slightly from a true ramp but the principle of operation will not be altered.

For very heavy load currents the transistor T2 may be replaced by more than one transistor, for example a Darlington pair may be used. This enables the current in resistor 8 and Zener diode 10 to be reduced.

The feature that the delay time can be adjusted without a substantial change in the total energy supplied to the load is of great value when the load is, for example, the operating coil of a print hammer 19 as shown diagrammatically in FIGURE 2. Before the operating coil is energised, the hammer armature is held against a stop by a restraining spring. Consequently, no movement of the armature occurs until the attractive force on the armature, due to the energisation of the coil, is equal to the restraining force exerted by the spring. This produces an initial delay between the start of energisation of the coil and the start of movement of the armature. Thereafter, the armature moves with a continuously increasing velocity, following a non-linear law, up to the time of impact of the hammer on the paper.

The initial delay period is increased by retarding the initial growth of flux in the operating coil. The total energy supplied to the coil is decreased as the delay is increased. However, the nonlinear velocity curve causes the proportionate increase in time of impact to be greater than the proportionate decrease in impact velocity. For example, a typical hammer mechanism was subject to a change in impact velocity of only 1-2% for a change in delay of -6%. The small change in impact velocity is of the right sign to increase equalisation of the velocity of a group of hammers. A hammer which needs to be delayed is likely to have a weaker restraining spring, or a more efficient magnetic circuit than the other hammers. In either case, this hammer will tend to have a higher velocity than the other hammers. Hence, the reduction of velocity consequent upon an increase in the delay will bring the velocity closer to the mean value for all the hammers.

We claim:

1. An electrical circuit including first and second power supply lines; an inductive load; a current limiting resistor and an electronic switch connected in series between said power supply lines; a source of first value of voltage connected between said power supply lines; means to apply an initiating signal to said electronic switch to change said switch from a non-conductive state to a conductive state; first means operative to maintain the voltage across the load at a substantially constant second value less than said first value of voltage for a period immediately following the change of the electronic switch from the nonconductive state to the conductive state; and said first means including second means operable to adjust the second value of voltage to control the rate of current rise in the inductive load.

2. An electrical circuit as claimed in claim 1 in which said. first means is rendered inoperative in response to the voltage across the current limiting resistor being equal to the difference between said first and second values of voltage.

3.- An electrical circuit as claimed in claim 2 in which the electronic switch is included in said first means.

4. An electrical circuit as claimed in claim 3 in which the electronic switch is operative in response to variation of voltage across the load to adjust the current flow through the switch to maintain said voltage at said second value.

5. An electrical circuit as claimed in claim 1 in which the first means includes a source of constant fixed voltage having a third value and said second means provides an adjustable stabilised voltage equal to the difference between said third value of voltage and the required second value of voltage.

6. An electrical circuit as claimed in claim 5 in which the second means includes a variable resistor and the electronic switch is operative to pass a constant current through said variable resistor.

7. An electrical circuit as claimed in claim 3 in which the electronic switch includes a transistor having base, emitter and collector electrodes; said emitter and collector electrodes being connected in series with the load and current limiting resistor; and including means operable to apply the initiating signal to the base electrode.

8. An electrical circuit in accordance with claim 1 including a transistor having base, emitter and collector electrodes; the emitter electrode being connected to the first supply line; the collector electrode being connected to the current limiting resistor; a first resistor and a Zener diode connected in series between the second supply line and the base electrode; and a feedback loop connected from the junction of the load and current limiting resistor to the base electrode.

9. An electrode circuit as claimed in claim 8 in which the feedback loop includes the Zener diode.

10. An electrical circuit as claimed in claim 1 in which the inductive load comprises an operating coil of a printer hammer mechanism.

References Cited UNITED STATES PATENTS 3,183,830 5/1965 Fisher et al. 317-1485 X 3,293,505 12/1966 Miller 317-1485 3,337,748 8/1967 Rusch et al.

LEE T. HIX, Primary Examiner US. Cl. X.R. 

